Chapter 12

PLATE TECTONICS

 

Introduction

Plate tectonic theory widely accepted by geologists today because it explains many features of the earth once thought to be unrelated; a unifying theory.  It help understand how Earth

has evolved and continue to do so.  The theory has evolved from numerous scientific inquiries and observations.

 

Plate tectonics was proposed in late 1960's by combining two earlier ideas: continental drift and sea floor spreading.

 

Continental Drift Hypothesis

Proposed by Alfred Wegener (fig. 12.2) in 1915; he suggested that the continents were once together and formed a giant supercontinent called Pangaea.

 

The data indicate Continental Drift occurred, but 2 major problems:

1. What is the driving force to move the continents? Wegener proposed both the Earth's rotation and drag of the moons orbit, but, these would move the plates in 1 direction

 rather than in all directions as the fit implies, and forces are not nearly great enough.

2. How can the rocks of the continents plow through rocks of the ocean? Violates what is known about the strength of rocks.

 

 

Evidence For Continental Drift:

1. Fit of continents- close resemblance of coastlines, particularly S. America and Africa; (best match along cont. slope at 2000 m depth) (fig. 12.3).

2. Similarity of Rock Sequences- same sequence of rock with same ages found on several continents  (fig. 12.4).

3. Continuity of Mountain Ranges- when continents are matched together, mountain ranges of the same ages are continuous between continents (fig. 12.5).

4. Glacial Evidence- till (rock formed from sediments deposited by glaciers) and glacial striations (scratch marks) indicate that glaciation occurred at same time on several continents;

if continents are joined and S. Africa moved to S. pole, extent and direction of movement of glaciers is consistent with how glaciers should form (fig. 12.6).

5. Fossil Evidence- same fossils (including land animals) found on continents now widely separated by oceans; Glossopteris (plant) (fig. 12.1)- seed too large to be transported by

wind and salt water would destroy; also won't grow in all of tropical to polar climates presently found in; Mesosaurus (freshwater reptile)- fossils found only in S. Africa and

S. America; couldn't live in salt water and if it could, it should be found in many other areas (fig. 12.7).

 

Wegeners ideas not taken too seriously because of the two problems mentioned above, also little additional evidence was found to support continental drift until 1950's.

 

6. Paleomagnetism and Polar Wandering- Earth has a magnetic field generated by electrical currents caused by different rotation speeds of mantle and liquid outer core; has lines

of force like a bar magnet (figs. 10.21, 10.22); magnetic poles aligned closely with geographic N & S poles; compass needle aligns with lines of force that 'flow' from S to N (fig. 10.22).

 

Rock Magnetism- some rocks rich in iron-rich minerals like magnetite (basalt); magnetite in a cooling magma aligns itself with Earth's magnetic field; when magma solidified

orientation frozen into place.

Aligned magnetic minerals indicate:

      1. direction of poles

2. inclination (fig. 10.22)- how steeply magnetic minerals dip; allows determination of latitude rock formed at; can tell if a rock/continent changed latitude over time.

 

From the study of iron rich lava flows of different ages, it was found that the location of the paleomagmetic pole apparently wandered over time- called Polar Wandering.

3 possible explanations for polar wandering:

      1. Earth's magnetic pole wandered over time & continents remained still

      2. Magnetic poles stationary & continents moved

      3. Both continents and poles wandered

 

How do we differentiate between these three possibilities?

      -look at polar wander paths for different continents

      -if paths match, poles wandered and continents stationary

      -if paths differ, continents or continents and poles wandered

      -found that each continent has its own path / magnetic pole; N. American and European paths differ, but are similar in shape (fig. 12.9); when these continents are fitted

together the paths match, indicating that these continents were together at one time!

 

Sea Floor Spreading

During the 1950's and 1960's, a renewed interest in oceanographic research led to mapping of ocean basins; important new findings during this period include: discovery of global

oceanic ridge system; high heat flow, volcanism, and seismicity at oceanic ridges; ocean floor composed of only young rocks (<180 my old); oceanic trenches with deep earthquakes.

 

-1950's, Harry Hess discovered guyots and determined that they indicated that the Sea floor was moving away from the oceanic ridges.

-in 1962, Hess wrote a paper that suggested that the Sea floor (as well as the continents) were moving; called this sea-floor spreading:

-his theory: oceanic ridges are located above upwelling portions of large thermal convection cells in the mantle; sea floor is carried in a conveyer-belt like fashion away from oceanic

ridge;    the oceanic ridge is pulled apart and filled by magma which solidifies and forms new oceanic crust (new oceanic crust is always being created - no wonder the ocean floor is

so young!);

downward parts of convection cells are located at oceanic trenches where oceanic crust is consumed.

 

Magnetic Reversals- at ~ same time Hess was presenting his ideas, it was found that the polarity of the Earth's magnetic field had flip-flopped over time (direction of lines of force

periodically reversed; get normal (present day) and reverse polarity).

1963, an important relation between magnetic reversals and sea floor spreading recognized:

-found alternating stripes of positive and negative magnetic polarity of rocks on the sea floor Parallel to the oceanic ridges; high intensity=normal polarity and low intensity=reversed polarity; basalt extruded at oceanic ridges is magnetized according to the existing magnetic field (N or R); get a mirror image of stripes of N & R magnetic polarity on either side of oceanic ridge that form these stripes (figs. 12.11, 12.14). When many dates from different parts of the ocean and continental lava flows were combined, a magnetic reversal time scale of N &R polarity with

corresponding dates of magnetic reversals was constructed (figs. 12.11); can determine age of sea floor anywhere by counting number of stripes from oceanic ridge! (like counting tree

 rings to determine age of tree).

By 1968, most geologists convinced plates on the Earth's surface moved; called this plate tectonics.

 

Plate Tectonic Theory:

Outer rigid lithosphere divided into several large and several smaller plates (fig. 12.14); rigid lithosphere overlies weaker ('plastic') asthenosphere; plates move as distinct units with

 ~all interactions occurring at plate boundaries.

 

The Supercontinent Cycle.  All continents come together to form the supercontinent (Pangaea).  Pangaea began fragmenting during the Triassic Period and continues to do so. 

A cycle spanning about 500 million years was proposed.

 

The 3 types of Plate Boundaries

Divergent Boundaries (spreading ridges)- plates moving apart; crust is extended, thinned, fractured, and basaltic magma rises into fractures forming new oceanic lithosphere; usually

occurs at crests of oceanic ridges but also occur within continents during the early stages of the splitting of a continent (fig. 12.16); fig. 12.15 shows typical history of a divergent plate

boundary.

 

Convergent Boundaries- if create new lithosphere at divergent boundaries and the Earth is not expanding, we must get rid of lithosphere somewhere- at convergent boundaries; one

 plate descends beneath another- called subduction; character of boundary depends on which types of plates are converging.

 

-ocean-ocean convergence- trench forms where one oceanic plate subducted beneath another; a Benioff zone of earthquakes is created; subducting plate partially melts and forms

magma that rises and is erupted from volcanoes; a volcanic island arc is formed (fig. 12.18a) (Japan is an example).

 

      -ocean-continent convergence- oceanic plate is always subducted beneath continental plate because it is more dense (cont. plate too buoyant to subduct);similar to ocean-ocean  

convergence except volcanism occurs on land (volcanic arc) (fig. 12.19a) (Pacific coast of S. America and pacific northwest of U.S. are examples).

 

      -continent-continent convergence- begins as ocean-continent convergence but when all oceanic crust is subducted, 2 continents collide; neither continent is subducted very far

 because of their low density; the edges of both continents are highly deformed and a large mountain range is formed (fig. 12.20a) (Himalayas, Alps, Appalachians are all examples).

 

Transform Boundaries- plates slide laterally past one another; lithosphere neither created or destroyed; earthquakes located along this boundary; usually connect 2 oceanic ridge

segments (fig. 12.22) (oceanic fracture zones); may extend into continents (like the San Andreas fault- fig. 12.23).

 

Plate Motion

Techniques for determining the rate of plate motion include:

1.      Age of sediments away from the spreading centers

2.      Use of magnetic anomalies in the ocean floor

3.      Satellite-laser ranging and GPS

Figure 12.24

 

Hot Spots- locations where stationary columns of magma originating deep within mantle (mantle plumes) rise to Earth's surface and form volcanoes (fig. 12.14); stationary so provides

us with a fixed frame of reference to determine absolute (instead of relative) movement of plates; Island of Hawaii is the youngest volcano along a long line of islands and seamounts that

have formed by moving over a stationary hot spot; allows us to track the movement of the Pacific plate (fig. 12.26).

 

Driving Mechanism of Plate Tectonics

One of the original problems people had with continental drift was lack of a driving mechanism; Hess suggested some type of mantle convection caused sea floor spreading and this is

still generally agreed upon.

 

Mantle Convection- result of unequal distribution of heat in Earth; warm mantle is less dense and rises at oceanic ridges; material spreads laterally and cools over time becoming more

dense; sinks back into mantle at trenches (subduction zones) where it is reheated.

Convection may occur in just the asthenosphere or throughout the entire mantle (or separate convection in both (fig. 12.27).

 

In addition to thermal convection, some geologists believe that plate movement may occur in part as a result of gravity:

            -slab pull- cold slab of lithosphere is denser than surrounding warmer asthenosphere and it therefore pulls the plate along (fig. 12.28).

            -ridge push- oceanic ridges are higher than surrounding oceanic crust, therefore gravity may push oceanic lithosphere away from ridges toward trenches (fig. 12.28).